This application discloses an invention which is related, generally and in various embodiments, to a modular multilevel converter (M2LC) system having a plurality of M2LC subsystems (cells) coupled to a current source power supply. The current source power supply is external to the M2LC subsystems and supplies control power to each M2LC subsystem independent of the energy state of the M2LC subsystem.
Many papers have been published regarding the Modular Multilevel Converter (M2LC) topology. FIG. 1 illustrates a two-level configuration of an M2LC cell having two terminals, and FIG. 2 illustrates a three-level configuration of an M2LC cell having two terminals.
As shown in FIG. 1, the M2LC cell includes two switching devices, two diodes, a capacitor and two terminals. With the configuration shown in FIG. 1, the two switching devices can be controlled such that one of two different potentials (e.g., zero volts or Vcap) may be present across the two terminals. As shown in FIG. 2, the M2LC cell includes four switching devices, four diodes, two capacitors and two terminals. With the configuration shown in FIG. 2, the four switching devices can be controlled such that one of three different potentials (e.g., zero volts, Vcap, or 2Vcap) may be present across the two terminals. Although other topologies of the M2LC cells are possible, all of the topologies may be defined as two-terminal subsystems or cells with internal capacitor energy storage(s) which are capable of producing various levels of voltages between the two terminals depending on the state of the switching devices.
It will be appreciated that the M2LC topology possesses the advantages of the Cascaded H Bridge (CCH) topology in that it is modular and capable of high operational availability due to redundancy. Additionally, the M2LC topology can be applied in common bus configurations with and without the use of a multi-winding transformer. In contrast to M2LC, CCH requires the utilization of a multi-winding transformer which contains individual secondary windings which supply input energy to the cells.
However, unlike CCH, the M2LC cells (or subsystems) are not independently supplied from isolated voltage sources or secondary windings. For a given M2LC cell, the amount of energy output at one of the two terminals depends on the amount of energy input at the other one of the two terminals. This can cause a problem in controlling the DC link voltages in these cells during pre-charge of the power circuit or during abnormal operation when one or more of the cells needs to be bypassed or made inactive.
Since during precharge, the operating voltage on the cell DC links can depend on the proper or improper operation of other cells connected in series, there exists a risk that significant system damage can occur before the cell power supplies can become active and allow the cell to communicate to a higher level controller (e.g., a hub).
Also, since the M2LC topology uses twice as many IGBT switches as its CCH counterpart, the M2LC topology is best suited to cell operating voltages which may be two or even four times the comparable CCH design to normalize or limit the power switch count. These higher bus voltages present a challenge to the design of the switch mode power supply that is usually part of the power cell to supply power to the gate control of the IGBTs and to the cell control circuits.
Furthermore, the M2LC topology allows for the shutdown of cells or portions of cells (in effect bypassing portions of the power topology) in which the cell control or gate control fails to operate correctly. However, because shutting down an M2LC cell causes the ultimate loss of the DC link voltage which supplies the cell power supplies, the shutting down of an M2LC cell causes the loss of cell control power.
Thus, it will be appreciated that utilizing a single power supply in each cell to provide the required cell control and gate control power to the respective M2LC cells is less than optimal under all operating conditions, including fault conditions.